Static Frequency Converters for Factory and Field Testing...

Static Frequency Converters for Factory and Field Testing of Power Transformers by Induced Voltages

Wolfgang Hauschild Andreas Thiede John Herron

Senior MemberHIGHVOLT Prüftechnik Dresden GmbH Reinhausen Manufacturing Inc.

Germany USA

Transformer Committee, Fall Meeting 2010,

Toronto, Canada, October 24 -28, 2010

Hauschild/Thiede/Herron Transformer Fall Meeting 2010

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1. Why Frequency Converters instead of Motor/Generator Sets ?

M/G based test system FC based system

Traditionally applied for stationary application, long experience;

Two-frequency M/G set or two separate sets;

Control also by means of power electronics, dynamics by the machines

Heavy machines for stationary application;

Special machines, difficult to get on the market; remarkable prices which may increase in the future.

Progress in power electronics enabled development, less experience till now;

Frequency can be adjusted (e.g. 40…200Hz, quartz-stable);

Excellent dynamic behavior, application of advanced control features;

Relatively lightweight and compact, optimum for on-site application;

With further development of power electronics even a reduction of price can be expected.


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2. Basic Problems: Sinusoidal Waveshape and Low PD Noise Level (1)

Frequency converters for drives are controlled according to a sinusoidal current, which means the waveshape of the voltage is not so important.

The IEC/IEEE Standards require a total harmonic distortion of

THD ≤ 5%,

The PD measurement shall be made down to

q ≈ 20 pC

which requires a PD basic noise level

qn ≈ 10 pC.

In HV testing and especially in loss measurement a sinussoidal voltage is required (IEC 60060-1, IEC 60076-3) –

necessary development of voltage controlled FC based on power transistors (IGBT).

Low THD means a large number of switching impulses of the IGBT’s, but low PD basic noise level means a low number of switching pulses –

necessary development of a FC which realizes the balance between THD and PD noise level.

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Some results of these developments are shown on the following transparencies.


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Symmetric disturbancesby pulsed output voltage (red: fundamental wave)

Asymmetric disturbances to ground

A sinusoidal waveshape within the IEC/IEEE requirements THD ≤ 5% can be guaranteedby internal filters in the FC and an external power sine wave filter acting against both types of disturbances.

No-load test: Voltage of THD=1.46%at current with THD=69%!


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In addition to special measures in the FC itself, the application of classical HV L-C filters between exciter transformer and transformer under test is

very useful.

Mobile system Stationary system

L

C


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The experience shows that industrial test labs with optimized HV test circuits can reach sufficiently low PD noise levels including test object :

qn ≤ 10pC.

3pC


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2. Induced Voltage Tests: The Requirements and the Solution (1)

Requirements:

Three-phase voltage

f ≥ 2 · frated = 100/120 Hz

THD ≤ 5%

PD measurement

Solution:

All requirements fulfilled, additionally:

Self-compensation is automatically adjusted;

No risk of self-excitation

Parallel connection of several systems


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2. Induced Voltage Test: Test Power Demand and Frequency (2)

apparent test power

test frequency

test voltage

Behavior of transformers under induced voltage test::

at lower frequencies – inductive load; at higher frequencies – capacitive load

in between “natural frequency” of self-compensation


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3. No-Load Loss Measurement : The Requirements and the Solution

Requirements:Three-phase voltage

f = 50/60 Hz

THD ≤ 5%

form factor (rms vs. rectified mean) ≤ 3%

Solution:All requirements fulfilled, additionally:

good dynamics due to rejection of test voltage harmonics



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4. Short Circuit Loss Measurement: The Requirements and the Solution

Requirements:

Three-phase voltage

f = 50/60 Hz

THD ≤ 5%

form factor (rms vs. rectified mean) ≤ 3%

Compensation of reactive power

Solution:All requirements fulfilled, additionally:

good dynamics due to rejection of test voltage harmonics


Negative sequence controller for single phase operation


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5. Example: Mobile Test System for On-Site Testing (1)

LV compensation Frequency converterStep-up transformer

Control room

HV filters,

voltage and current measurement,

PD coupling capacitors

Design of a trailer with all equipment necessary for testing with induced voltages


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5. Example: Mobile Test System for On-Site Testing (2)

In addition to the test requirements, mobile systems are characterized by

low weight-to-test power ratio,compact,robust against mechanical shocks during transportation,low power demand,easy to handle.

FC-based test systems are optimum for these requirements.


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6. Example: Stationary Test System for Routine and Type Testing (1)

Frequency converters, filters, primary compensation, water cooling

Capacitor bank for compensation, step-up transformer top, busbars

Dresden Transformer Test Field of Siemens AG

FC characteristics: 1250 kW, 6000 kVA

Capacitor bank: 133 000 kVA

Step-up transformer; steps up to 80 kV

Photos: Courtesy Siemens AG TBD


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6. Example: Stationary Test System for Routine and Type Testing (2)

Dresden Transformer Test Field of Siemens AG

Variable frequency resonant test system 350 kV for applied voltage tests, Impulse voltage test system 2000 kV,200kJ, Systems for induced voltages as described.

Photos: Courtesy of Siemens TBD


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7. Conclusions

1. For power transformer testing, AC voltage test systems based on static frequency converters are successfully operating for both, routine and type testing in factories and field testing after assembling or repair on-site.

2. All requirements of the relevant standards can be fulfilled. Power rating can be enlarged by the application of the modular principle of parallel connections of several systems. This is especially helpful for transformer testing in the field.

3. The remaining present limitation in active power of 4 MW and in apparent power of 12 MVA will be enlarged with the further development of power electronics.

4. The authors have no doubt that transformer testing will be dominated by systems based on power electronics in the future.

Static Frequency Converters for Factory and Field Testing...

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